What version of photoshop and operating system are you using?
Are your wanted colors on a different layer?
When in the Replace Color dialogue - the color picker acts only in "Current Layer " mode (regardless of the setting of the eye dropper outside of replace color) . So you can only sample colour from that layer.
Thanks Dave. You nailed it. It was indeed an issue of attempting to sample a color from a different layer. Well now I know!
I found it strange as most other tools will respect the setting of the general eye dropper tool.
Now here is a bizarre workaround for you :
1. Go to Window - Arrange - New Window for xxxxxxxx (your document). This will open the same document in a second window
2. Then Window Arrange Tile 2 up vertically (so you see both versions)
3. Open replace color and click on the color you want to replace
4. Click on the Result box to open the color picker
5. In the first image window click on the image and hold whilst you drag the eye dropper onto the color you want in the duplicate image window then release
6. The color picker will update with that wanted color from the duplicate window